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I have a Hydor 300w inline running on my 75g tank. It keeps the temps constant and have no complaints. If you go with traditional heaters you might want to consider getting two heaters since discus like higher temps. That way if one fails at least you have something until you get a replacement.
Another consideration is that many plants don't like the high temps of a discus tank. You will need to choose carefully to get varieties that will thrive.
The two (smaller) heater idea is a good one, as you are also less likely to cook your fish if one fails.